The development of sensitized water based explosives and techniques for loading ANFO in small diameter holes was prompted by the industry's desire to eventually eliminate nitroglycerine from their product lines. However, the sensitized water based explosives and ANFO generally lack the degree of sensitivity that is intrinsic to nitroglycerine based compositions. Both ANFO and the water based explosives are less sensitive to initiation and in the case of the water based explosives they have a tendency to become less sensitive at low temperatures. When initiated under border-line energy conditions, ANFO and the water based explosives are prone to detonate inefficiently and in some cases to fail. With most nitroglycerine sensitized explosives, the standard practice is to initiate with a No. 6 blasting cap. ANFO cannot be initiated reliably in this manner. Although some water based compositions are sensitive to a No. 6 blasting cap and are sometimes used in this fashion, the industry recognizes the need for a small primer with substantially greater power output for application with most non-nitroglycerine explosives.
A variety of explosive packages for use as primers is known in the art as disclosed, for instance, in U.S. Pat. Nos. 3,037,452; 3,212,438; 3,276,372; 3,371,606; 3,401,632; 3,431,849; 3,431,851; 3,614,928; 3,604,353 and 3,747,527.
The industry has developed and is marketing miniature plastic explosive primers for the intended use. The weight of these units varies up to 20 grams. The units are designed to fit snugly over a blasting cap. Some units are closed at one end, to prevent extending the base of the cap completely through the plastic explosive. If the base of the cap is extended through and beyond the plastic explosive, the cap will fail to initiate the primer. Other units are open at both ends for use with detonating cord or with a cap providing that the leg wires of the cap are threaded through the primer to hold the cap in place. However, the latter does not assure that the base of the longer caps cannot be extended beyond the primer.